The present invention relates to anodizing as well as the production of anodized aluminum.
The term anodized aluminum as here used refers to aluminum and its alloys which have been subjected to the anodizing process to produce adherent aluminum oxide (Al.sub.2 O.sub.3) coatings thereon. Such aluminum oxide coatings provide hard and strong protective coatings for the soft surface of aluminum and its alloys and are formed on parts made of aluminum and its alloys to protect same against corrosion and abrasion, to strengthen such parts, to provide electrical insulation thereon, and in some instances for the purpose of providing decorative effects.
Heretofore, the typical manner by which an aluminum or aluminum alloy part has been anodized has been to immerse the part in a tank containing an anodizing electrolyte, such as sulfuric acid, sulfamic acid, oxalic acid, chromic acid or phosphoric acid, and causing a continuous direct current (DC) to flow through the electrolyte between the part and the tank, the part being the anode in this electrolytic cell formed and the tank being the cathode. Further, it has been found that by refrigerating the electrolyte to lower its temperature from ambient temperature, i.e. 68.degree.-72.degree. F., to a relatively cold temperature, the hardness of aluminum oxide coating formed on the part is increased. Accordingly, the anodizing process conducted with the refrigerated or cold electrolyte has been commonly referred to as hard anodize.
The above-discussed prior art anodizing process has several drawbacks. Firstly, since the aluminum or aluminum alloy part has to be immersed in the electrolyte, the tank has to be at least as long as the part. Thus, to anodize long parts, long tanks have to be built and to hard anodize such long parts a substantial quantity of electrolyte must be refrigerated. These requirements make the cost of hard anodizing long parts quite expensive. Additionally, it is apparent that it is inefficient to use a relatively long tank for hard anodizing a short part since energy will be wasted, for example, in refrigerating the large quantity of electrolyte contained in the long tank. Thus, it may be necessary to custom make anodizing tanks and custom design refrigeration plants to handle specific parts to be anodized which have different lengths. All this is costly.
Secondly, the prior art anodizing process has the disadvantage of not being suited for use in a continuous process system. Since the part to be anodized is totally immersed in the electrolyte, no other work can be simultaneously performed on the part while it is being anodized.
Additionally, the thickness of the aluminum oxide coating which presently can be produced by the prior art anodizing process is relatively thin while the power and time consumed in producing same is relatively high. Also, the prior art anodizing process has the disadvantage that the current density of the DC anodizing current is relatively restricted and must be carefully controlled in order to avoid burning of the part being anodized. As a consequence, since the electrical resistance of the part to the anodizing current flow increases as the thickness of the aluminum oxide coating formed thereon increases, the DC voltage applied to cause the flow of anodizing current therethrough must be gradually increased from a relatively low level in a controlled manner during the anodizing process in order to insure the formation of the aluminum oxide coating without burning. As a consequence, it has generally been necessary to employ a skilled operator to monitor and control the operation of the prior art process.